Means to control cross talk



Aug 13, 1929. E, GREEN 1,724,082

MEANS TO CONTROL CROSS TALK Filed June 26, 1928 Rectifier .DemnalalatorINVENTOR El. GI eeI/u ATTqRNEY Patented Aug. 13, 1929. A

lJNITED STATE-S I 1,724,0 2 PATENT OFFICE.

ESTILL I. GREEN, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO AMERICAN TELE-PHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

MEANS TO CONTROL CROSS TALK.

Application filed June 26, 1928. Serial No. 288,459.

This invention relates to transmission circuits, and more particularlyto arrangements for reducing cross-talk or interference upon suchcircuits.

In designing telephone transmission circuits with respect to cross-talklimitatlons, there are two factors to be taken intoconsideration,--first, the eifect of cross-talk upon the intelligibilityofthe conversation while talking is taking place, and second, theabsence of secrecy or the annoying eliect of the cross-talk during gapsin the conversation. The latter factor determines the limitingcondition, for the reason that the maximum cross-talk current whichwould be permissible without rendering the conversation unintelligiblewill be greater than the maximum cross-talk which may be permittedwithout rendering such cross-talk understandable or. objectionabledurlng gaps in the conversation.

It has heretofore been proposed to ehminate or reduce cross-talk ornoise currents from external sources during such intervals .asconversation is not taking place, by providing switching arrangementsfor normally disabling the circuit for transmlsslon puroses, suchdisabling means being removed, however, under the control of volcecurrents when conversation takes place.

Such a switching arrangement for shortcircuiting or opening the circuitdur ng idle periods, in the manner above described, 1s not, however,entirely practicable for use under -all conditions which are encounteredin practice. It is thereforeproposed, in accordance with the present1nvent1on,. to arrange the receiving apparatus of the carrier channelsso that the receiving galn Wlll be appreciably less at small inputs thanat normal speech loads, by providing what is in efiect a purelyelectrical switching arrangement. This results in the receivingapparatus discriminating against cross-talk during non-talkingintervals, xwhile per nltting of proper transmission of the relatively-large talking currents.

The invention may now be more fully understood from the followingdescription, when read in connection with the accompanying drawing,Figure 1 of which shows the invention applied to an amplifier of areceiving circuit, and Fig. 2 of which shows the invention applied tothe detecting arrangement of a receiving c rcuit.

Referring to Fig. 1, an amplifier comprisng a Vacuum tube A andassociated circuits 1S connected by means of transformers 11 and 12between circuits L and L The ampl fier may be used either as anamplifier of voice currents, or it may be included in one of thereceiving channels of a carrier or radio system, in which case thecircuit L may extend to apparatus connecting with the main line orantenna upon which the channels are superposed, while the circuit L mayextend to the demodulating apparatus associated with the particularchannel in question.

In order to control the effectiveness of the amplifier, a by-passcircuit is bridged across the input circuit of the amplifier tube A,said by-pass circuit including a glow discharge tube, such as, forexample, a neon tube N,

a resistance 14 and a battery 15. The battery 15is of such voltage andis so poled as to normally break down the resistance of'the neon tubeand cause a glow discharge to pass so that the neon tube, beingconductive, will enable the bypass circuit to shunt the input of theamplifier and thereby cut down its gain.

In order to control the effectiveness of the by-pass, some of theincoming energy from the circuit L is applied to a rectifier tube Bthrough .a transformer 13. The rectifier tube R has its output circuitconnected to a resistance '14; in the bypass, so that when current flowsin the output circuit of the rectifier, as the resultof applied signalenergy, a drop in the resistance 14 is set up in such a direction as tooppose the voltage of the battery 15 and thereby. render the glowdischarge tube non-conductive. The by-pass thereby becomes in effectopen-circuited, so that the amplifier operates at its normal gain.

, The usual grid biasing battery 16 is included in the grid circuit ofthe amplifier,

and the voltage of this battery is such as to apply the roper bias tothe grid tocause the amplifier to operate most efliciently as anamplifier'when the by-pass is in efiect open-circuited. The potential ofthe grid is determined in part by the battery 15 which causes the glowtube to discharge, and in part by the biasing battery 16.

When signals of small energy, such as 7 those corresponding to-noise orcross-talk, are beingrecelved, the rectified current flowmg in thereslstance 14 is such as to oppose the potential of the batteryinsufliciently to prevent the glow of the discharge tube N, so that theamplifier will be ineffective to transmit such signals. Waves of largeamplitude, corresponding to speech, however, as already stated, causesuflicient opposing potential to the battery 15 to in effect opencireuitthe by-pass. Thussubstantially no input energy will be by-passed throughthe tube N, so that the system will have maximum gain.

If desired, a filter F may be included in the output of the rectifier toprevent the transmission through the resistance 14 of frequenciesnormally occurring in the audio frequency band, while permitting therectified current to change in amplitude with suflicient rapidity sothat no appreciable portion ozt-a'syllable will be lost when the talkerstarts to speak. When the filter F is used, with its consequent delay inbuilding up the current in resistance 14, a network F, having anapproximately equivalent delay should be included in the circuit L atthe point illustrated, to enable the current to be I built up in theresistance 14 before the signal wave is actually impressed upon the gridcircuit of-the amplifier.

Fig. 2'shows the principle above described applied to the demodulator ofa receiving channel of a carrier or radio system. The demodulatorcomprises two vacuum tubes'D and D connected in a push-pull circuit andinterconnected between circuits L and L thrqugh transformers 11 and 12,respectively. In the usual type of carrier system, the carrier issuppressed at the transmitting station and pnl the side bandtransmitted. Consequently, t e demodulator at the receiving station hasassociated'with it a source 0 for supplying alternating current of thecarrier frequency, this source being connected to the common branch ofthe grid circuit through a transformer 17.

A by-pass is connected across the input circuit of the demodulator tubeD, the said by-pass comprising a glow discharge tube N, a resistance 14and a battery 15 for rendering the glow dischar e device conductiveSimilarly,-a by-pass is ridged across the input circuit of the tube D,comprising a glow discharge tube N connected in series with thereslstance 14, and battery 15 of the other by-pass, thebattery 15serving, of course, to'cause the tube N to discharge. A biasing battery16 is also connected in the common branch of the grid circuit to applysuch a potential to the grids of the tubes D and D as-will render themeffective demodulators when the by-passes are in effect open-circuited.

As in thecase of Fig. 1, some of the signaling energy from the circuit Lis applied to the rectifier R whose output circuit is connected so as tocause a drop due to the rectified direct current through the resistance14', this drop being in such a direction as to oppose the'potential ofthe battery 15 and tend to render the glow discharge tubesnonconductive. For weak received currents corresponding to noise orcross-talk, the drop through the resistance 14 is insufiicient tointerrupt the discharge of the tubes N and N, so that the noise orcross-talk energy applied to the modulator through the transformer 11is, to a large extent, by-passed and prevented from effectivelyoperating the demodulator to produce audible signal energy in thecircuit L When currents of large amplitude, such as those correspondingto voice signals, are received, however, the drop through the resistance14 is large enough to oppose the potential of the battery 15 to such anextent as to render the tubes N and N non-conductive so that the bv-passbecomes in effect open-circuited and the demodulator supplies to thecircuit L a maximum signal current.

It will be obvious that the general principles herein disclosed may beembodied in many other organizations widely different from thoseillustrated, without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

1. In a transmission system, a transmission circuit, receiving apparatustherefor, and means to render the receiving gain of such apparatusappreciably less in response to smallreceived current than in responseto currents corresponding to normal signals, said means comprising aconductive shunt bridged across said receiving apparatus, and meanscontrolled by received currents to vary the'conductivity of said shunt.

2. In a'transmission' system, a transmission circuit, receivingapparatus therefor, and means to render the receiving gain of suchapparatus appreciably less in response to small received current than inresponse to currents corresponding to normal signals, said meanscomprising a normally conductive shunt bridged across said receivingapparatus, and means controlled by received currents to render saidshunt less conductive for lar received currents than for small receivecurrents.

3. In a transmission system, a transmission circuit, receiving apparatustherefor and means to render the receiving gain of such apparatusappreciably less in response to small received current than in responseto currents corresponding to normal signals,

d. lln a transmission system, a transmission circuit, receivingapparatus therefor, and means to render the receiving gain of suchapparatus appreciably less in response to small received current than inresponse to currents corresponding to normal signals; said meanscomprising a circuit bridged across said receiving apparatus andincluding a normally conductive glow discharge tube, and meanscontrolled by received current to change the conductivity of said glowdischarge tube.

5. ln a transmission system, a transmission circuit, receiving apparatustherefor,

and means to render the receiving gain of such apparatus appreciablyless in response to small received current than in response to currentscorresponding to normal signals, said means comprising a circuit bridgedacross said receiving apparatus and including a normally conductive glowdischarge tube, and means controlled by received cursion circuit,receiving apparatus therefor,

and means to render the receiving gain of such apparatus appreciablyless in response to small received current than in response to currentscorresponding to normal signals, said means comprising a circuit bridgedacross said receiving apparatus and including a normally conductive glowdischarge tube, means to produce a direct current whose amplitude isdetermined by that of the received current, and means controlled by saiddirect current to produce a potential in said shunt in such direction asto oppose the conductivity ofsaid glow discharge tube.

lntestimony whereof, l have signed my name to this specification this21st day of June, 1928.

ESTILL ll. GREEN.

